Culture of the African Animal-Associated Members of the MTC
The optimal in vitro growth conditions for most animal-associated members of the African lineage of the MTC are poorly defined. The chimpanzee bacillus has been grown on Lowenstein-Jensen (LJ) media and on Middlebrook 7H9 and 7H11 media supplemented with Oleic Albumin Dextrose Catalase (OADC) (Coscolla et al.
2013). The growth rate of this organism appears to be similar to that of M. africanum in it being somewhat slower than the growth of M. tuberculosis (Coscolla et al. 2013; Gehre et al. 2013).The other members of this lineage display very slow in vitro growth. When first isolated in 1954, samples of the dassie bacillus were incubated for 4 months on Dorset’s egg medium before growth was first detected (Wagner et al. 1958; Wagner and Bokkenheuser 1961). The difficulty in growing this organism was confirmed by Cousins et al. (1994) who reported that it grew poorly on egg and agar-based media but that growth was stimulated by pyruvate. Culture of the dassie bacillus from diseased tissues has also been achieved using the mycobacterial growth indicator tube (MGIT) system (Becton Dickinson, New York, NY, USA) (Parsons et al. 2008). However, several months of culture may be required to detect growth.
The MTC isolate originally recovered from banded mongooses, and subsequently named M. mungi, was grown on LJ slants with and without pyruvate (Alexander et al. 2002). Only a few acid-fast colonies of this organism were present after 6 weeks of incubation on this medium. Similarly, the culture of tissue homogenates from 52 meerkats with suspected TB for 10 weeks on LJ slants supplemented with either pyruvate or glycerol resulted in positive MTC growth in only 42% of cases (Drewe et al. 2009a,b). Cultures of M. suricattae have been established from tissue homogenates using the MGIT system; however, subsequent subculture on Middlebrook 7H10 agar for 8 weeks resulted in growth of only one of four of these isolates (Parsons et al. 2013). These results highlight the need for extended culture times of 2-3 months, or longer, for isolation of these species.
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